The present invention relates to a process for separating two elements adhering one to the other by means of adherent faces, with this separation being able to be obtained under the action of a fluid and/or a mechanical element allowing the separation to be initiated locally. It also relates to a device for implementation of this process.
The invention applies in particular to the field of micro-electronics in order to separate two plates adhering one to the other. It is of particular interest in handling thin, fragile and very flexible plates.
Document FR-A-2 752 332 discloses a process for separation of a support plate by the insertion, at the bonding interface, of a quite flexible separator element so as not to scratch the surfaces. This separator element consists of several parts allowing minimum optical sighting, during the operation to open the interface, to be compatible with an industrial goal. This process was developed for plates bonded by means of attractive forces.
The article entitled xe2x80x9cBonding of silicon wafers for silicon-on-insulatorxe2x80x9d by W. P. MASZARA et al., published in the review J. Appl. Phys. 64 (10), 15 Nov. 1988, pages 4943-4950, relates to the measurement of bonding energy by the method of the blade inserted at the interface of two elements adhering one to the other. For a given bonding energy, the thicker the blade, the further the opening wave propagates from the opening point at the bonding interface. Similarly, the greater the bonding energy, the less the separation wave propagates for a given blade thickness.
To apply a separation process as described in document FR-A-2 752 332, it is advantageous if the surface energy is low and the separator element is thick. A separation wave may thus be propagated over a significant length compared to the diameter of the plates for separation.
However, use of a thick separator element may lead to a fracture of one of the plates due to the curvature radius being too low. In addition, it has been shown that the greater the bonding energy, the more the blade or the separator element must be inserted gently in the interface to prevent the risk of fracture of the plates, relaxation of the opening stresses being made possible through a sufficiently slow opening.
In addition, in the case of a structure with several interfaces, the opening may be propagated from one interface to another, associated, for example, with a lower bonding energy.
It is also known that the bonding energy between two elements increases when a thermal treatment is applied. On this subject one may refer to the article by C. MALEVILLE et al., published in the review Electrochemical Society Proceedings Volume 97-36, pages 46-55. As an a example, silicon plates the surfaces of which have been made hydrophilic are bonded to one another. A bonding energy greater than 1 J/m2 is obtained for bondings followed by thermal treatment at 1000xc2x0 C. Thus, for silicon plates 525 xcexcm thick (typical thickness of plates of 100 mm diameter), a blade 600 xcexcm thick succeeds in causing an opening of the bonding interface over a length of around 3 cm or less. This length of opening is insufficient to separate the plates. It is then necessary to introduce a thicker separator to propagate this opening. This causes a reduction of the flexibility of the separator elements and involves the risks mentioned above.
Inserting a blade is not the only method enabling two elements bonded to one another to constitute a structure to be separated. Document WO 98/52 216 describes a process for controlled cleavage of a substrate through the introduction of particles, originating, for example, from a steam source, from a side of the structure where the interface ends. However, this technique can be used only to separate stacks in which a zone has previously been embrittled, for example by ion implantation. The separation interface can then only be the embrittled zone. U.S. Pat. No. 5,863,375 discloses the separation of two plates bonded to one another to constitute a structure. Separation is obtained under the effect of a jet of liquid directed on the plane of the interface to a face of the structure where the interface ends.
Moreover, the faces of the plates for separation may have received, before being bonded, one or more deposits of thin films. In this case it is not possible to use the teaching of U.S. Pat. No. 5,863,375. The separation liquid jet also acts on deposited films.
As there is no precise location of the bonding interface, the separation may occur in one of the deposited layers if the adherence energy of a film to its plate is less than the adherence energy of the bonding interface between the two plates. This technique is also very expense in terms of the consumption of fluid used, since a large quantity of this fluid does not act on the bonding interface.
These known techniques for separation using a jet of particles or a jet of liquid replacing a separating blade reveal other problems. A first problem relates to the precise location of the opening interface. Other problems are related to the fact that to apply the opening techniques easily the bonding interface must not be too resistant taking account of the various thermal treatments which can be applied.
Traditionally, the bonding energy may be controlled by preparing the surfaces to modify their hydrophilic character or their roughness. On this subject, one may refer, for example, to the document xe2x80x9cinfluence of surface characteristics on direct wafer bondingxe2x80x9d by O. Rayssac and coll., 2nd international conference on materials for micro-electronics, 14/15 Sep. 1998, ION Communications Ltd.
Document EP-A-0 703 609 discloses a process for transferring a thin semiconducting layer from a support substrate to a target substrate, taking advantage of the fact that the bonding energy between the layer and the support substrate is less than the bonding energy between the layer and the target substrate. When a pulling and/or shearing and/or torsion force is applied to the structure, the separation occurs between the layer and the support substrate, thus causing the layer to be transferred.
This process must, as above, take account of the possible problem of resistance of the bonding interface.
In addition, the thin layer is bonded to the support substrate in order to undergo a number of processes including, for example, one or more deposits of thin films the adherence energy of which may prove to be lower than the bonding energy of the substrates to each other. In particular, methods of separation based on traction, shearing or torsion, applied globally to the substrates, may not be used.
The invention has been designed to remedy the disadvantages reported above.
To this end, the invention relates more specifically to a process for separating two elements of a structure containing both elements put in adherent contact with one another by respective adherent faces and with at least one interface.
Before adherent contact is accomplished, the process involves at least one cavity being made. The cavity is made in at least one of the elements, ending respectively at the interface, to allow separators to pass into the cavity. The process also comprises, during separation, the exercise of a force, in a localised manner in the interface, through the application of the said separators to initiate the separation of the two elements from the interface and to continue it, if applicable, until complete separation of the two elements.
The separators may include, among other things, means exerting a mechanical action and/or fluid pressure and/or exerting a chemical action on at least one of the adherent faces at the interface.
Thus, the force applied to the interface must be understood as resulting from a mechanical and/or fluid pressure and/or chemical action.
The cavities may be obtained by engraving. They may be made on the periphery or in a more central region of the elements. In particular they may be distributed across all or part of an interface of adherence between the elements, so as to control propagation of the separation opening. The cavities may also extend as far as an interface separate from that formed by the elements"" adherence faces, inside one of the elements.
If several interfaces are used for the separators, the cavities may be arranged so as to initiate the separation at a given location of the interfaces.
When the separators include means exerting a fluid pressure in the interface zone, and when this fluid is liquid, these separators may include microwaves or impulse excitation of the liquid fluid.
The two elements may be put into adherence with one another with an adherence energy which varies according to the various regions of the adherence interface so as to initiate the separation at a given location of the interface.
In addition, the separators may be such that the separation of the two elements in the interface occurs at one or more locations in a simultaneous or sequential manner.
The invention also relates to a device for separating two elements of a structure, adhering to one another by adherence faces at least one of which has cavities in an interface zone, so as to be able to subject at least one of the adherence faces to the influence of a fluid and possibly to a mechanical action, with the device comprising an enclosure with at least a first chamber called a high-pressure chamber able to receive the fluid, and a second chamber, called a low-pressure chamber, the enclosure being formed so as to receive both the adherent elements of the structure such that the cavities communicate with the high pressure chamber.
This device may also include means forming a stop to a deformation considered excessive of one and/or the other element of the structure on their separation.
The enclosure may, preferably, be fitted with at least one joint arranged between an element of the structure and the wall of the enclosure to separate the high-pressure chamber from the low-pressure chamber.
The invention also relates to a handle for transferring objects such as, for example, electronic chips. The handle has an adherence face with cavities in at least one interface zone, to which objects may adhere, and the handle also has means of access to the interface zones to separate objects from them. The objects are of various dimensions, from a few microns to several tens of centimetres for example.
The means of access are, for example, channels or any other type of depression or perforation made in the adherence face.
The handle may contain a plate one face of which constitutes the adherence face, with the plate being pierced with penetrating holes ending in the interface zones and constituting the said means of access to the interface zones. The penetrating holes may be holes allowing a tool for separating objects to be passed through.
The means of access to the interface zones preferably allow a fluid pressure to be applied to the objects.
A fourth goal of the invention relates to a process for localised transfer of objects made on the surface of a first substrate, with an adherence face, where the process comprises the following stages (in this order or in another order):
putting the adherence face of one or more object in adherent contact with the adherence face of a transfer handle as described above.
possibly, thinning of the first substrate from the free face of this first substrate,
placing into adherent contact of at least one of the said objects with a receiving substrate, and
separation of the said object from the handle using the means of access to the interface zone,
The process may be completed by the separation of the transfer handle, which may contain objects which are not yet transferred, and of the receiving substrate which contains the transferred objects.
If the objects are not separated from one another, i.e. clipped before the placing into adherent contact with the receiving substrate, the process may also include a stage of clipping of objects so as to allow their individual transfer.